1. Field of the Invention
The present invention relates to an image forming apparatus in which an electrophotographic system is adopted, such as copiers, printers or facsimile machines, particularly relates to an image forming apparatus in which a magnetic brush developing system using a two-component developer comprising a toner and carrier is adopted.
2. Description of the Related Art
In image forming apparatuses in which an electrophotographic system is adopted, such as computers, printers or facsimile machines, an electrostatic latent image corresponding to image information is formed on a photosensitive body constituting an image carrier as disclosed for example in Laid-open Japanese Patent Application No. 2000-227690 and this electrostatic latent image is converted to a visible image by toner acting as developer within a developer device so that a toner image is obtained. In execution of this development process, in an image forming apparatus of this type, in view of considerations regarding for example toner charging stability or charging performance and development performance, the magnetic brush development system using a two-component developer comprising a toner and carrier is often employed. This magnetic brush development system is a system in which the toner is charged up by friction between the toner and carrier, a magnetic brush is formed on a developing roller constituting a developer carrier, and development of the latent image on the photosensitive body is performed by means of this magnetic brush. The magnetic brush attaches charged toner to spikes formed from the carrier. The developing roller comprises a magnet roller comprising a non-magnetic sleeve and a plurality of magnetic poles that are arranged within this developing sleeve: a magnetic field such as to cause the developer to sprout up in the form of spikes is formed on this developing sleeve. The developer, in which these spikes are created on the sleeve surface, moves by movement of this developing sleeve or the magnet roller. The developer on the sleeve that is conveyed in the developing region facing the photosensitive body causes spikes to sprout up along the lines of magnetic force generated from the developing magnetic poles of the aforementioned magnet roller. The developing agent that causes this sprouting up in the form of spikes produces a visible image by contacting the surface of the photosensitive body in a grazing fashion and supplying toner in respect of the electrostatic latent image in an amount based on the relative linear speed ratio of the photosensitive body and the developing roller.
In an image forming apparatus of this type, for example the DC bias development system, in which development is performed using DC voltage or the DC/AC bias development system, in which development is performed by superimposing an AC voltage on a DC voltage, is employed. In the DC/AC bias development system, fine line reproducibility can be improved and gradation reproducibility can be improved by using the AC voltage to produce vibration of the toner in the space in the developing region; also, with the DC bias development system, development performance in terms of toner deposition performance onto the photosensitive body is inferior to that in the case of the DC/AC bias development system. It has therefore come to be recognized, as mentioned in for example Laid-open Japanese Patent Application No. 2004-133178, that images of better image quality can be obtained with the DC/AC bias development system than in the case of the DC bias development system.
However, regarding the ground contamination characteristic of the photosensitive body, the DC bias development system is much better as regards this degree of ground contamination than the DC/AC bias development system. This is because, compared with the DC/AC bias development system, in which the toner tends to fly off during cycling between the developing roller and the photosensitive body, due to superimposition of an AC voltage on the DC voltage, in the DC bias development system, there are far fewer opportunities for the toner to fly off, since the electrical field acts unidirectionally from the developing roller to the photosensitive body.
Furthermore, in recent years, in image forming apparatuses of this type, instead of pulverized toner, polymer toner has come to be employed, that is manufactured by a polymerization method and that provides high image quality and high transfer efficiency due to its narrow particle size distribution and uniform shape. In the case of image forming apparatuses using polymer toner, reproducibility of fine details is sometimes better in the case of the DC bias development system than in the case of the DC/AC bias development system, so it cannot necessarily be said that the DC/AC bias development system is always better in terms of image quality. Also, the DC bias development system, in which only DC voltage is employed and AC voltage is not employed, has the considerable user merit that costs can be lowered due to the fact that a DC power source is not required. It is therefore expected that image forming apparatuses adopting the DC bias development system and employing polymer toner will become predominant in the future, since they combine advantages in terms of both image quality and costs.
However, the DC bias development system has unsatisfactory development performance compared with the DC/AC bias development system. Efforts are therefore being made to compensate for this by improving the development process and the developer. These require for example increase of the linear speed ratio Vs/Vp of the developing sleeve linear speed Vs and the photosensitive body linear speed Vp, or raising of the toner density. However, although making the linear speed ratio Vs/Vp large improves development performance, it results in a severe so-called edge effect, in which the amount of toner deposited at the edges of the electrostatic latent image on the photosensitive body is increased. Specifically, the so-called image density loss phenomenon is generated, in which peripheral portions of block regions or halftone regions in the image are emphasized and portions further on the outside thereof lose image density. For example, if block regions are present in the halftone regions of the image, the halftone portions in the peripheral portion of the block regions are not developed, due to the edge effect, and so lose image density.
It is also possible to improve development performance by making the developing gap, which is the gap between the photosensitive body and the developing sleeve in the developing region, narrow. However, although development performance is improved by reducing the developing gap, the sliding frictional force with the photosensitive body produced by the magnetic brush is augmented, resulting in a severe edge effect as described above, with the result that loss of image density occurs due to the edge effect. This has side-effects including loss of image density at the rear edge of the image, deterioration of reproducibility of transverse lines, and decreased life of the developer due to increased stress on the developer. Furthermore, although decreasing the developing gap improves the development performance, sliding frictional force when the magnetic brush applies pressure to the photosensitive body is increased. As a result, since the amount of toner that is held by the spikes of developer that contribute to development is reduced, the phenomenon of toner that has already been developed being, contrariwise, scraped off the photosensitive body after development i.e. the “reverse development” phenomenon occurs. As a result, blurring of the rear edge portion of the image or loss of image density of peripheral portions of block images occurs. In particular, in cases where the arrangement is such that the angle of the main magnetic pole of the developing roller and the photosensitive body becomes 0°, impact with the photosensitive body takes place in a condition with the developer spikes standing out therefrom in a location in which the developing gap is at its narrowest, so a considerable amount of toner is scraped off. Consequently, the problem arises that the rear edge portion of the image is blurred or the phenomenon of loss of image density of the peripheral portion of block images occurs, resulting in considerable loss of image quality.
Furthermore, as mentioned for example in Laid-open Japanese Patent Application No. 2003-240065, in recent years, in which the developing gap has been further narrowed, unevenness of density of development caused by minute vibrations of the developer carrier such as the developing roller have tended to become more noticeable. The reason for this is that the degree of variation of the width of the developing region produced by such vibration and the instantaneous rate of change of developer performance increase as the developing gap is narrowed. Consequently, even if misalignment of axes of the rotary shafts of the gears in the drive transmission system of an image forming apparatus and the various process devices is cancelled, unevenness of development density tends to occur due to minute vibration of the developer carrier caused by factors other than such misalignment of axes. For example, although minute vibration of the developing sleeve was caused by meshing impact of the teeth of adjacent gears provided to drive this developing sleeve, in conventional devices, in which the developing gap was set comparatively wide, the unevenness of developing density produced by such vibration was not particularly noticeable. However, in recent years, in which the width of the developing gap has been continually reduced, unevenness of developing density produced by such vibration has become serious. A further problem is that unevenness of developing density is also produced by transmission to the image carrier of minute vibration caused by meshing impact of the teeth of adjacent gears in the drive transmission system of other image forming units, such as the photosensitive body, provided in the vicinity of the image carrier.
Technologies relating to the present invention are also disclosed in, e.g., Laid-open Japanese Patent Application No. 2003-255627.